I did a 50% water change of the 130 gallon tank. Today I wanted to clean the DIY filter. As I was trying to drain it, a LOT of the bad water & gunge spilled into the tank. I tore apart the filter, rinsed out the tub I use to hold the pot scrubbers/floss. I put it back together, added a sheet of black foam all the while I was draining the tank of another 50%.I just now did a test & nitrates are still high. I didn't rinse the scrubbers out with tank water. I do have an amazon sword in the tank.

@Sandman1969 wrote:100% water change over 2 days should lower it, but it hasn't.

It's impossible that it didn't unless you have nitrates in your tap water.

Nitrate is in the tank water, and if you replace it with water that doesn't have nitrate in it, you dilute it.

I assume by what you are saying, you aren't seeing a change in the water test. Two possible theories for that. First the test could be a dud. If it's a liquid test, you really need to shake the heck out of the two solutions (I mean like really shake it, bang it around, 30 secs +). The second thought could be that the nitrate levels were off the charts in the water, and that diluting by half, and half again, it's still off the chart.

Just to clarify, two 50% changes of water also doesn't equal a 100% change of water in the tank. Just as an example, if you had 100ppm of nitrate in the water (a ludicrously high number, but simpler to do the math on), and you did a 50% change, you would have 50ppm remaining. You fill the tank back up. The next day you do another 50% change. You still have 25ppm of nitrate in the tank.

Keep hammering the water changes, nitrates will come down. Chewing thru Prime is a kick in the wallet. Switch to the powder form, Seachem Safe. 1/4 tsp will treat 300g of water. If BigAls dont have it, Petsandponds does. $40 for a 1 kilo jug, will last years. No expiry. If your filter was excessively dirty, that in itself will become a nitrate bomb. Shorten the timeframe for filter cleaning

Gunk and debris buildup in the bottom of the tub is still a bad thing. All water flowing thru the filter comes in contact with this slop, and in turn toxins are being leached back into the tank. I would almost guarantee this is contributing to the nitrate puzzle. You need more mechanical filtration, or more frequent cleaning of the tub filter

You have a large volume tank with big fish= the need for excessive filtration. Adding biomax is fine but wont address the need for more turnover and extra mechanical filtration. I suggest adding a pair of Aquaclear 110 hobs. Set them up as strictly mechanical. The sponge it comes with in the bottom, perhaps a finer sponge on top of that, pack the rest with floss. Easy to rinse, and will extend the timeframe of having to open the tub filter.

The pump for the tub filter is a 600+ GPH pump. Is that not enough for this size?I do have a few other HoBs hanging around. An old biowheel filter without wheel & an Aqueon.

As for those 110s, I can't afford $100 a shot for that.

The foam I mentioned earlier was just added as it never had it before. The tub only had the floss & scrubbers. Would that make any difference?The tank also has one of those 2 sponge air filter units that's powered by an air pump, if that makes a difference.

The actual flow rate of any filter is reduced by as much as 40%, given both types of media in the filter and some degree of debris trapped in sponges, floss etc. By your own admission, the filter was holding a lot of waste. I would venture an estimate of 350-400gph tops. Thats a realistic 3x tank turnover. Not near enough for a heavy bioload tank. The recommendation for big tanks/big fish is 5x turnover for canisters and 10x for hobs. Understood that you cant lay out money for more big filters. The dilemma is though, you need significantly more mechanical filtration. If you have hobs kicking about, by all means hook them up. Maybe keep an eye on Kijiji, perhaps a good filter at a manageable cost shows up.